Dosage compensation is the equalization of most X-linked gene products in males where a single X chromosome and a single dose of X-linked genes are present and in females where two X chromosomes and two doses of these genes are present. In Drosophila, this equalization is achieved by enhancing the level o transcription of X-linked genes in males. Therefore, dosage compensation is a unique model system with which to study transcriptional regulation in eukaryotes. Furthermore, because of its position as the target of well-characterized regulatory genes that also control sexual differentiation, the study of dosage compensation makes an important contribution to the understanding of this fundamental developmental pathway. Our experimental goal is to understand the underlying regulatory mechanisms responsible for dosage compensation at the molecular level. The recent cloning of the four regulatory genes that we have identified as the regulators of compensation represents a significant breakthrough toward achieving this goal. Some of the characteristics of these genes suggest a very interesting parallel with a newly described multi-protein complex o yeast and the possibility that we may be on the verge of demonstrating the existence of a eukaryotic mechanism for transcriptional regulation that may be more wide-spread than was here-to-fore-recognized. In the present application we propose the following specific aims: (1) We will complete the characterization of the known regulatory factors responsible for transcriptional enhancement in males, including the cloning and product characterization of a new gene that we have recently recovered in a mutation screen of the X chromosome. This aim involves not only the characterization of individual factors but also a thorough study o their interactions. (2) There is experimental as well as circumstantial evidence for the existence of additional genes contributing to the molecular mechanism of X hyperactivation. We will continue to search for and characterize new regulatory genes by means of special mutational and molecular screens. (3) We will determine the basis of the sex-specific control mechanism that normally limits the occurrence of dosage compensation to males. The master regulatory gene Sxl controls the occurrence of dosage compensation but the steps that mediate this control are not known. We will investigate the nature of these steps.